Statistical Properties of Cold Streams In Massive Star-Forming Halos in IllustrisTNG-50

TL;DR

This paper addresses how cold, dense streams penetrate massive halos during cosmic noon ($z \sim 2$–$4$) and how often they reach the central galaxy. It introduces a novel HDBSCAN-based pipeline to automatically detect stream-like gas in the IllustrisTNG-50 simulation and systematically quantify their prevalence, temperature, density, and mass flux across $z=4$–$0$. The results show that cold streams are ubiquitous in halos with $M_h \gtrsim 10^{12} M_\odot$ from $z\gtrsim4$ down to $z\sim1$, peaking around $z\sim2$ with on average about three streams per halo and a characteristic co-planar geometry; however, in TNG-50 these streams typically disrupt before penetrating to the galactic disk, with maximum penetration near $\sim 0.6$–$0.8\,R_{200c}$. The study highlights the need for higher-resolution, CGM-focused zoom-in simulations to resolve mixing layers and feedback–inflow interactions that determine whether cold streams can sustain star formation by feeding the disk.

Abstract

Cold, dense streams of gas are predicted to penetrate deeply into massive halos (> 10^12 Msun) at cosmic noon (z=4-2), fueling galaxies to sustain high star formation rates. We investigate the prevalence of such cold streams in IllustrisTNG-50 over the range z=4-0, using a novel algorithm to automatically detect cold streams in simulated halos. We qualitatively and quantitatively characterize the geometric and physical properties of the detected streams over cosmic time. We find that cold streams are ubiquitous in massive halos at cosmic noon, occurring in more than 80 percent of such systems down to z=1, before becoming rare by z=0. At their peak prevalence (z=2-1), streams are often found in roughly co-planar, three-stream configurations. These streams generally exhibit a dense and cool core, surrounded by a diffuse and warmer envelope. However, we find that in IllustrisTNG-50, these streams typically disrupt in the outer halo and do not penetrate efficiently to the central galaxy, with the total mass inflow from streams peaking at z=2. Our results underscore the importance of cold streams in fueling galaxies at early times, but they highlight the need for higher-resolution simulations to fully capture their survival and impact at later epochs. Future cosmological zoom-in simulations, with better resolution in the CGM, will be essential to resolve turbulent mixing layers and feedback-inflow interactions that determine whether cold streams can reach the galactic disk.

Figures (13)

• Figure 1: Schematic diagram of the stream identification algorithm. In the first panel, we start with all gas particles in a subbox centered around the halo. In the second panel, we select gas within $0.15-3\,R_{200c}$ from the halo center and filter for cold stream-like gas. In the third and fourth panels, we illustrate two examples of the clustering algorithm. In the fifth panel, we demonstrate the merging algorithm. In the sixth panel, we illustrate the final stream selection based on geometric properties of the candidates.
• Figure 2: A selection of 20 halos with $M_h \geq 10^{11.95} M_{\odot}$ at $z=2$ in IllustrisTNG-50. For each of the halos, we show a mass flux projection plot, with a depth of $0.1\,R_{200c}$, where blue indicates inflowing mass and red indicates outflowing mass. $R_{200c}$ is demarcated via the dashed black circles for each halo, and each map extends $2.6 \,R_{200c}$ on each side to show the broader environment of the halos. At the bottom of each map, we list the halo ID in the simulation as well as the halo mass. These halos demonstrate the prevalence of streams in these types of systems, as well as the diversity in the configurations and properties of the streams.
• Figure 3: Two example halos at $z=2$ that exhibit cold streams, halo 10 ($M_h = 10^{12.81} M_{\odot}$; top row) and halo 120 ($M_h = 10^{12.04} M_{\odot}$; bottom row). We show a surface density map (left column), mass flux map (middle column), and density-weighted temperature map (right column), with a projection depth of $0.1\,R_{200c}$. For each map, $\,R_{200c}$ is demarcated with the inner circle, and $2.5\,R_{200c}$ with the outer circle. Like in Figure \ref{['fig:Halo_Gallery']}, each map extends $2.6\,R_{200c}$ on each side to show the broader environment of the halos.
• Figure 4: Hammer projection maps corresponding to Figure \ref{['fig:halo_examples']}. We show a surface density hammer projection (left column), mass flux hammer projection (middle column), and the mass flux hammer projection masked to highlight strong mass influx (right column), with a projection depth of $0.1\,R_{200c}$ around $r = R_{200c}$.
• Figure 5: Prevalence of streams in massive halos over redshift. Left: Percentage of halos with $M_h \geq 10^{11.95} M_{\odot}$ that have at least one stream as a function of redshift. Right: The mean number of streams in a halo, taken over only halos with at least one stream (purple-dashed line) and over the entire halo population (pink-solid line) as a function of redshift.